1. Field of the Invention
This invention relates to centrifugal separator apparatus for separating particulate material from an air stream, and especially to high efficiency, horizontal, twin-flow, two stage separator apparatus with minimum air pressure drop. The centrifugal separator apparatus is particularly useful for removing particulates from the gaseous discharge from drying equipment such as large capacity, multi-stage, horizontal rotary dryers before return of the dryer exhaust gases to the atmosphere.
2. Description of the Prior Art
It has long been the practice to remove the moisture from various agricultural, industrial and by-product materials by passing the moist material through a rotary drum dryer, either of the multiple stage, multiple pass or multiple stage single pass type. Burner gases at an elevated temperature are directed through the dryer to vaporize a substantial part of the product water content and to also serve as a conveying medium for the product along the length of the dryer. Often times, a certain proportion of the dried material is recycled back to the drum dryer in order to decrease the overall moisture content of the product input to the dryer, thus enhancing the dryer""s efficiency.
The gaseous discharge from the rotary dryer must be treated before being returned to the atmosphere in order to lower the amount of particulates entrained in the air stream to meet regulatory standards. In recent years, governmental agencies have imposed increasingly stringent regulations on the amount of particulates that may be discharged into the surrounding atmosphere from large scale drying equipment. Particulate removal has been accomplished for the most part by directing the particulate bearing exhaust gases from large scale, industrial sized rotary dryers into one or more upright cyclones. Although cyclones are functionally capable of substantially reducing the particulate content of a gas stream containing entrained particulate materials before return of the gas stream to the atmosphere, upright cyclones of requisite efficiency are relatively expensive, require a significant footprint area in the vicinity of the horizontal dryer, and work most effectively when two substantially identical cyclones are employed in side-by-side serial air flow relationship.
An exemplary dryer and associated cyclone separator is shown and described in my U.S. Pat. No. 4,193,208 of Mar. 18, 1980. As illustrated in FIG. 1 of the ""849 patent, a burner assembly is provided that burns natural gas or a similar fuel feed stock to produce hot products of combustion which are directed into the inlet end of an elongated, generally horizontal, hollow drum heat exchanger rotatable about its longitudinal axis. The negative pressure inlet of a centrifugal discharge and fan unit is connected to the outlet end of the drum dryer for inducing flow of relatively high volumes of air through the dryer in association with the hot products of combustion from the burner assembly. The positive pressure outlet end of the fan unit is connected to the inlet of an upright cyclone collector which discharges the substantially particulate-free air back into the atmosphere.
A conveyor at the discharge end of the dryer receives dried product and directs that product to a point of use or for further drying. Similarly, product removed from the air stream directed into the cyclone gravitates from the lower end of the cyclone vessel and may, if desired, be combined with the product output from the dryer.
Another exemplary horizontal rotary dryer, fan unit and cyclone separator is described and shown in my U.S. Pat. No. 5,157,849 issued Oct. 27, 1992.
This invention concerns a horizontal, negative pressure centrifugal separator for removing particulate material from an air stream that exhausts from the outlet end of a horizontal industrial size, rotary dryer. The centrifugal separator is adapted to be connected to the negative pressure inlet of a fan assembly which functions to pull large volumes of air through the dryer drum.
The centrifugal separator preferably comprises an essentially horizontal drum having wall structure presenting two side-by-side primary material separation plenum chambers of generally spiral configuration. The primary plenum chambers mutually communicate with a central plenum chamber therebetween which is also of spiral configuration.
In one form of the centrifugal separator, frusto-conical, opened-ended, pressure regain divider stacks are positioned between each of the separation plenum chambers and the central plenum chamber. In this embodiment of the centrifugal separator, each of the separation plenum chambers has an air inlet and the central plenum has an air discharge outlet. The divider stacks each have a generally conical section which is joined to an annular stack component presenting a central aperture. The divider stacks extend into the central plenum chamber with the apertures thereof in horizontal, generally axially aligned, facing relationship.
Air containing entrained particulate material entering the air inlets of the separation plenum chambers follows a generally serpentine path within respective separation plenum chambers and then exits the plenum chambers into the central plenum chamber via the apertures in corresponding divider stacks. The conical configuration of the divider stacks, along with the annular stack component coaxial with the axis of a respective stack, which causes each of the stacks to have pressure regain properties, serves to minimize the pressure drop in the air flow therefore preventing significant air pressure loss during operation of the separator. The air flow through the centrifugal separator of this invention is approximately 40% greater with substantially equal pressure drop as compared with conventional cylindrical stack separators.
Material removal plenums of generally spiral configuration are also provided at opposite ends of the separator drum and communicate with corresponding separation plenum chambers. Each of the material removal plenums is provided with a material discharge opening, preferably located at the lower portion of a respective material removal plenum. The centrifugal force exerted on the air streams during flow along respective spiral paths causes particulate material in the air streams to migrate toward the spiral inner surface of the separation plenum chambers. Particulate materials separated from the two air streams directed simultaneously into the separation plenum chambers collects in the outer material removal plenums and eventually is discharged from the outer material removal plenums through the material discharge openings in lower portions of respective material removal plenums.
In another form of the separator, the central spiral plenum chamber has either one or two air inlets while the spiral separation plenum chambers on opposite sides thereof each have an air discharge outlet. In this form of the separator, the divider stacks are oriented such that they extend away from each other and into corresponding separation plenums. An air stream containing particulate material that must be removed from the air before discharge of the air back into the atmosphere is directed into the air inlet of the central plenum chamber, commences flow in a generally spiral direction within the central plenum chamber, passes through the central aperture in respective divider stacks, flows along spiral paths within the separator plenums, and is discharged through the air outlet openings of the two separation plenum chambers. In this embodiment, particulate material displaced from the air streams by centrifugal force is received in the spiral material removal plenums and gravitates therefrom through the discharge openings of the material removal plenums.
The horizontal separator having side-by-side, simultaneously operable separation plenums which communicate with a central plenum provide a separator which is substantially as effective in removing particulate material from an air stream as a conventional upright cyclone, without occupying as much space as that cyclone and at an advantageous capital cost. In addition, the horizontal disposition of the separator permits air having particulate material entrained therein to be introduced into the separator at any one of a number of different circumferential locations, and to allow for discharge of cleaned air at any point around the circumference of the separator drum. This permits the separator to be connected between the outlet of the dryer and the primary fan at an optimal position, with a minimum overall footprint.
A horizontal separator in accordance with this invention, having side-by-side dual stage separator plenums with associated pressure regain stacks is capable of removing as much as 96% or more of particulate material in a stream of products of combustion and air discharged from a horizontal industrial size product dryer. Thus, by positioning the separator between the primary fan and the dryer, and connecting the fan to one or more conventional cyclones, the level of particulates ultimately discharged into the atmosphere may be maintained at a very low level and one that meets regulatory standards therefor.